This invention relates to an X-ray generator having an improved cathode.
The conventional X-ray generator has a hot cathode which is typically made of tungsten whose operational temperature is very high as 2000 to 2300 degrees Celsius. Other than the tungsten, thorium-added tungsten or lanthanum hexaboride also have been used for the hot cathode materials. The operational temperature of those materials is 1000 to 1500 degrees Celsius which is lower than that of the tungsten but is a relatively high temperature. The hot cathode made of any materials described above requires a relatively high-power heating power supply. The hot cathode made of thorium-added tungsten or lanthanum hexaboride requires high vacuum to obtain a steady emission current. The tungsten filament requires vacuum under 1xc3x9710xe2x88x923 Pa, while the hot cathode made of thorium-added tungsten or lanthanum hexaboride requires vacuum under 1xc3x9710xe2x88x925 Pa.
Since the conventional X-ray generator has a hot cathode as described above, it has the following disadvantages: (1) With the hot cathode, a high-power heating power supply is required. A large current (e.g., ten and several amperes) must flow through the hot cathode to emit hot electrons and thus a large-current cable is required. Since a negative high voltage of several tens kV based on the ground potential is supplied to the cathode of the X-ray generator, a cable connected to the X-ray generator must bear not only a high-voltage but also a large current and heat generation. Such a large-current high-voltage cable is expensive, thick, rigid and difficult to handle. (2) Since the cathode becomes a very high temperature, the surrounding parts must be designed to bear the high temperature. (3) The cathode made of lanthanum hexaboride and so on requires high vacuum. (4) The hot cathode becomes a high temperature to discharge gas which affects the X-ray generator. Therefore, before the use of the X-ray generator, the hot cathode must be heated for a period of time to discharge gas so as to reduce gas discharge in the actual use. (5) The cathode material would slightly evaporate and scatter from the hot cathode, so that such material adheres to the target surface and causes contamination with which the characteristic X-ray of the adhering material (i.e., cathode material) is generated inadvantageously.
Incidentally, in the field other than the X-ray generator, carbon nanotubes have lately attracted attention as a cold cathode electron emission source. The carbon nanotube is one form of carbon material which has a cylindrical structure with a diameter of nanometer order. The carbon nanotubes can emit electrons by field emission under the room temperature even with the flat surface of the electron emission region (i.e., requiring no needle shape). It is known that the cold cathode electron emission source made of carbon nanotubes may be used for the electron source of the flat display, as disclosed in Japanese patent publication Nos. JP 11-194134A (1999), JP 10-199398 A (1998), JP 10-149760 A (1998) and JP 10-12124 A (1998). The cold cathode electron emission source emits electrons which collide against fluorescent substance to make a light-emitting display. Also it is known that, the carbon nanotubes may be used for the electron gun of the cathode ray tube, as disclosed in Japanese patent publication Nos. JP 11-260244 A (1999) and JP 11-111158 A (1999).
Furthermore, it is known that, other than the carbon nanotubes, fullerenes may be used for the cold cathode electron emission source, as disclosed in Japanese patent publication No. JP 10-149760 A (1998), the fullerene being another form of carbon material.
Accordingly it is an object of the invention to provide an X-ray generator in which a cold cathode electron emission source made of carbon material is used as the cold cathode so that various problems caused by the hot cathode can be solved.
It is another object of the invention to provide an X-ray generator in which a cold cathode electron emission source made of carbon material emits electrons which heat a hot cathode so that a high-voltage cable is given no large current.
An X-ray generator according to the first aspect of the invention includes a cathode having an emitter made of carbon nanotubes which emits electrons by field emission and thus becomes a cold cathode electron emission source. In the invention using the carbon nanotubes, any one of the following three forms is adopted to control the tube current apart from the electron-focusing control. The first form is that a takeoff electrode is disposed near the cathode and the Wehnelt potential and the takeoff electrode potential are controlled independently. The second form is that an electron emission source is disposed behind the cathode and the electron emission source emits electrons which collide against the back of the cathode so that the cathode temperature is controlled in a range of the room temperature to about 100 degrees Celsius to regulate an amount of electron emission from the cathode. The third form is that the cathode has a heater so that the cathode temperature is controlled in a range of the room temperature to about 100 degrees Celsius to regulate an amount of electron emission from the cathode.
The emitter made of carbon nanotubes has the following advantages as compared with the conventional hot cathode: (1) Since the cathode requires no high-temperature heating, it saves power. (2) The cathode requires no large-current cable which is used for high-temperature heating. (3) Since the cathode temperature is near the room temperature, the surrounding parts requires no countermeasure for a high temperature. (4) Since the cathode has no high-temperature region, it requires no heating operation for outgassing before the use so that the X-ray generator can be used soon. (5) If the cathode becomes a high temperature, the cathode material would evaporate and adhere to the target surface. The cathode of this invention has no such a problem and the target contamination is reduced. (6) A steady emission current is obtained under a pressure of about 1xc3x9710xe2x88x923 Pa so that the X-ray generator requires no high vacuum.
Fullerenes may be used instead of the carbon nanotubes. The fullerene has a polyhedral structure including pentagons and hexagons made of carbon atoms, the typical one being a spherical structure including 60 carbon atoms. Such fullerenes may be used for the cathode emitter of the X-ray generator.
An X-ray generator according to the second aspect of the invention includes a hot cathode and a cold cathode electron emission source made of carbon material (e.g., carbon nanotubes) for heating the hot cathode. The hot cathode is not a direct-heating type in which a current directly flows through the cathode to heat it by resistance, but a type in which electrons from the electron emission source collide against the cathode to heat it. The carbon nanotubes are used as the emitter of the electron emission electrode. The electron emission electrode is disposed behind and apart from the hot cathode. The electron emission electrode is -given a negative potential based on the hot cathode potential so that the electron emission electrode emits electrons which collide against the hot cathode to heat it. The negative potential is controlled to regulate the tube current of the X-ray generator. The hot cathode material is not limited to specific ones, but at least an electron emission region is made of lanthanum hexaboride preferably. Fullerenes may be used instead of the carbon nanotubes.
The X-ray generator according to the second aspect of the invention includes a hot cathode heated by electrons which are emitted by a cold cathode electron emission source made of carbon material (carbon nanotubes or fullerenes), so that a high-voltage cable is given no large current.
The X-ray generator according to the second aspect has the advantage described below as compared with that according to the first aspect. It is known that an electron emission surface made of carbon nanotubes generates uneven brightness and its hourly fluctuation, the uneven brightness of the emitter being that an electron emission strength depends upon locations on the electron emission surface. It is desirable in the X-ray generator that uneven brightness on the target is reduced as much as possible and hourly fluctuation of the X-ray intensity is reduced as much as possible, the uneven brightness on the target being that a strength of electron current colliding against the target depends upon locations on the target surface. Therefore, if the carbon nanotubes are used as the cathode as in the X-ray generator according to the first aspect, the above-described uneven brightness of the emitter and its hourly fluctuation would affect the performance of the X-ray generator. The X-ray generator according to the second aspect has no such problem.